1. Field of the Invention
The present invention relates to an apparatus for extracting basic shape patterns or primitive patterns included in an image such as a character or any other figure, the image primitive pattern extraction apparatus being suitably applied to an image recognition apparatus such as an OCR (Optical Character Reader).
2. Description of the Prior Art
In conventional image recognition for a character or a figure, a feature extraction process for class classification is performed to discriminate an input image subjected to image recognition from other images and to define a class which is assigned to the input image. The feature extraction process for class classification is an indispensable operation in image recognition since the input image is eventually recognized on the basis of the results of class classification. The features are extracted by a feature extraction apparatus in an image recognition system.
Conventional feature extraction apparatuses are classified into an apparatus using mainly electronic techniques and an apparatus using mainly an optical system.
In a conventional feature extraction apparatus using the electronic techniques, an input image is converted into an image signal, and the image signal is stored in a memory. The image signal is then preprocessed and is subjected to projection processing, thereby extracting the features of the image.
A typical example of the feature extraction method adopting the optical procedure is an optical correlation method generally called as an image casting. Such method is described in the published book (ex: Topics in Applied Physics vol 48., Edited by S. H. Lee, P97, Springer-Verlag). This method uses many optical channels to extract a feature included in a optical pattern image. That is, a microlens forms the optical pattern image on a primitive mask. A light beam passes through a part of the primitive pattern in the primitive mask where the optical pattern image overlaps with the primitive pattern in the primitive mask. This beam is detected by a sensor. If a shape of feature included in the optical pattern image needs to be extracted, the many optical channels must be used to cover all regions of the optical pattern image with many primitive patterns in the primitive masks where each primitive pattern is fabricated in different positions in each primitive mask.
Another typical example of the feature extraction apparatus using the optical system uses image conversion by means of Fourier transform. Such a feature extraction apparatus will be generally described.
A Fourier transform optical system is used in the feature extraction apparatus. The Fourier transform optical system includes a high-resolution projection lens (objective lens) having a predetermined focal length. An object plane (i.e., an input image plane) and an image formation plane are formed on the front and rear focal planes of the objective lens, respectively.
A transmission image (an input image) recorded on a photographic film is located on the object plane, and monochromatic collimated beams are incident on this image. The input image is focused by the objective lens as a Fourier-transformed image on the image formation plane. Therefore, the resultant image is the one obtained by frequency-converting (Fourier transform) the input image.
A matched filter is positioned on the image formation plane. This filter is a mask of a Fourier-transformed image corresponding to a feature to be extracted from the input image. Another high-resolution convex lens (restoration lens) is located in front of the matched filter and is spaced apart therefrom by the focal length of the lens.
Only a pattern beam having the same feature as that of the matched filter, among the pattern beams of the Fourier-transformed image of the input image passes through the matched filter. The Fourier-transformed image of the input image is subjected to so-called spatial filtering. The spatially filtered Fourier-transformed image is inverse Fourier-transformed by the restoration lens to form an image on a second image formation plane. Only an image pattern having a basic shape pattern or a primitive pattern corresponding to the matched filter, among the input image patterns, is formed on the second image formation plane.
If a photosensor is arranged such that its light-receiving surface is aligned with the second image formation surface, the beam passing through the matched filter is condensed by the restoration lens and is incident on the light-receiving surface of the photosensor. By detecting an output of the photosensor, it is discriminated whether the input image has a predetermined primitive pattern. Hence, the input image can be classified.
In the conventional technique using image conversion by means of Fourier transform, however, several special-purpose components such as two high-resolution lenses for Fourier transform, inverse Fourier transform, the matched filter, coherent light source and the like are required. In addition, the input image must be accurately located at a predetermined position and in a predetermined direction.
At the time of assembly and maintenance of the apparatus, high-precision operations are required that cause cumbersome operations. In particular, special care must be paid for alignment of the input image. For example, if the input image is angularly deviated by .theta., the deviation is reflected on the frequency plane and hence causes rotation of the Fourier-transformed image through .theta.. The Fourier-transformed image of the input image which is designed to be aligned with the matched filter pattern is not actually aligned therewith. Feature extraction cannot be performed in practice. In addition, the optical system is undesirably complicated.
The Fourier-transformed image of the input image is repeatedly compared with a large number of matched filters to extract the features. The feature extraction time is undesirably prolonged.